If you thought the days were over when you could come up with some inane social app and get millions of Facebook users, think again.

For weeks, an app called "I want to add your birthday" from a company called K-Factor Media has hovered in AppData's top 10, with more than 25 million monthly active users. It's more popular than Spotify on Facebook.

A second, identically named version of the app is rising fast, with 23 million monthly active users. That's more than Yahoo or Pinterest's Facebook-connected apps.

If you're wondering what the point of a birthday-tracking app on Facebook is, you've hit the nail on the head: The "I want to add your birthday" app does nothing but track friends' birthdays—you know, the way Facebook already does.

It's not clear if that's the same app, but we also noticed an earlier version of "I want to add your birthday" in our list of blocked apps. So it seems likely that this birthday-app gambit is nothing new—it's just come back with a vengeance.

Birthdays are an integral part of the Facebook experience. It's a tradition among users to post on each other's walls on their birthdays. Facebook has taken advantage of that to boost e-commerce on the site by encouraging users to buy each other gifts as well. Anything which distracts users from Facebook's own birthday announcements could hurt those efforts.

While Facebook apps were famously spammy in the early days of the platform, the company has gotten a lot better at policing spam.

We asked Facebook why this app persisted, attracting tens of millions of users.

"This app is not in violation of any of our policies, and there aren't enough people marking it as spam for it to trigger our detection systems," a spokesperson told us. "If an individual finds the app to be spammy, they can hide or remove the app stories from their News Feed or block the app."